Nowadays, the number of functions with which an electronic part is endowed has been increasing. As a result, more and more electronic parts using semiconductor devices are being adopted for use in household electric equipment, information equipment, communication equipment, industrial equipment, etc. The semiconductor devices which are used in such electronic parts are extremely sensitive to an abnormal voltage such as static electricity or a lightning surge voltage. Accordingly, it is not unusual to cause a malfunction of the associated electronic apparatus or the destruction thereof. Thus, it is very important to overcome the problem of surge voltages in these semiconductor devices so that the associated electronic apparatus may have a higher degree of reliability.
To cope with this problem, a surge absorber has conventionally been used. Such a surge absorber, however, is short-circuited when it malfunctions as a result of deterioration, etc., so that there is the danger of its being overheated and ignited. To avoid such an accident, a number of measures have been proposed.
FIGS. 36 and 37 show the construction of a conventional surge absorber. The surge absorber shown includes a surge absorbing element 1, and electrodes 2a and 2b provided on both sides thereof. The surge absorber also includes elastic leads 3 and 4, which are connected to the electrode 2a by means of low-melting-point solders 5, and a lead 6, which is connected to the electrode 2b by means of a high-melting-point solder (not shown). The leads 3, 6 and 4 are connected, respectively, to the connection terminals 8, 9 and 10 on a supporting member 7.
The operation of this surge absorber, constructed as described above, will now be described. FIG. 37 shows the way in which this surge absorber is used. The connection terminals 8 and 9 are connected to a power source 11, and the connection terminals 9 and 10 are connected to a circuit 12 which is to be protected. Normally, the section between the connection terminals 8 and 10 is short-circuited, and the section between the connection terminals 8 and 9 has such a high resistance that they are isolated from each other. When a surge voltage is generated in the power line, the resistance of the surge absorbing element 1 is lowered, and the section between the connecting terminals 8 and 9 becomes nearly short-circuited, with the result that the surge current does not flow through the circuit 12 to be protected. Instead, the surge current flows between the connecting terminals 8 and 9, thus allowing the surge to be absorbed. When, in this circuit, a continuous excess voltage is applied to the power line, a continuous excess current flows to the surge absorbing element 1. As a result, the surge absorbing element 1 is heated and fuses the low-melting-point solder 5, so that the elastic leads 3 and 4 are separated from the electrode 2a, as indicated by the arrow in FIG. 38. Accordingly, the surge absorbing element 1 and the circuit 12 to be protected are isolated from the power source 11. Thus, this surge absorber acts as an electronic part having a safeguard function.
The problem with this conventional construction is that, when a continuous excess current flows through the surge absorbing element 1 and causes the low-melting-point solders to melt so as to separate the leads 3 and 4 from the electrode 2a, which have been connected to the latter, an arc discharge due to the high voltage may occur between the electrode 2a and the leads 3 and 4 which have been separated therefrom. Thus, the surge absorbing element 1 cannot be perfectly isolated from the excess current, which means that there is still the danger of the surge absorbing element 1 being ignited.